Use AJAX submit when confirmation form is modal

Hi,

I think maybe this change doesn't take proper account of all the possible configurations that a Flag can have. Specifically, an AJAX error is thrown when flagging a node for 'dialog'-style flags.

Steps to reproduce

• Set up a basic Drupal site using `drupal/recommended-project` eg. as here https://docs.ddev.com/en/latest/users/quickstart/#drupal
• Create a new 'Content' Flag type, choosing 'Dialog' as the 'Form behavior'
• Create and publish a new Basic Page node
• View the new node
• Click the 'Flag this item' link
• Click the 'Create flagging' button in the dialog that appears

The flag state changes on the page but the dialog box doesn't close and an AJAX error is visible in the browser console:

`An error occurred during the execution of the Ajax response: Error: cannot call methods on dialog prior to initialization; attempted to call method 'close'`

I don't understand the lifecycle of the dialog component well enough to propose a proper resolution, but a quick-and-dirty patch to check for a behavior type of 'modal' as well as for an XMLHttpRequest context when deciding whether to add the `#ajax` key to the submit button render array has stopped the issue occurring for me (albeit at the expense of a page reload vs. the nicer intended AJAX behaviour).

Of course - no problem. Here it is:

https://www.drupal.org/project/flag/issues/3625010